fix: gemini system prompt capture

Author: carlos-marchal-ph

posthog/ai/utils.py

@@ -173,14 +173,28 @@ def merge_system_prompt(kwargs: Dict[str, Any], provider: str) -> List[Formatted
 
         contents = kwargs.get("contents", [])
         formatted_messages = format_gemini_input(contents)
-
-        if kwargs.get("system_instruction") is not None:
-            system_instruction = kwargs.get("system_instruction")
-            system_message = cast(FormattedMessage, {
-                "role": "system", 
-                "content": system_instruction
-            })
-            formatted_messages = [system_message] + list(formatted_messages)
+        
+        # Check if system instruction is provided in config parameter
+        config = kwargs.get("config")
+        system_instruction = None
+        
+        if config is not None:
+            # Handle different config formats
+            if hasattr(config, "system_instruction"):
+                system_instruction = config.system_instruction
+            elif isinstance(config, dict) and "system_instruction" in config:
+                system_instruction = config["system_instruction"]
+            elif isinstance(config, dict) and "systemInstruction" in config:
+                system_instruction = config["systemInstruction"]
+        
+        if system_instruction is not None:
+            has_system = any(msg.get("role") == "system" for msg in formatted_messages)
+            if not has_system:
+                system_message = cast(FormattedMessage, {
+                    "role": "system", 
+                    "content": system_instruction
+                })
+                formatted_messages = [system_message] + list(formatted_messages)
 
         return formatted_messages
     elif provider == "openai":

posthog/test/ai/test_system_prompts.py

@@ -231,7 +231,7 @@ def test_gemini_contents_array_system_prompt(self):
             self._assert_system_prompt_captured(properties["$ai_input"])
 
     def test_gemini_system_instruction_parameter(self):
-        """Test Gemini with system_instruction parameter."""
+        """Test Gemini with system_instruction in config parameter."""
         try:
             from posthog.ai.gemini import Client
         except ImportError:
@@ -256,10 +256,10 @@ def test_gemini_system_instruction_parameter(self):
             client = Client(posthog_client=self.client, api_key="test")
 
             contents = [{"role": "user", "content": self.test_user_message}]
+            config = {"system_instruction": self.test_system_prompt}
 
             client.models.generate_content(
-                model="gemini-2.0-flash", contents=contents,
-                system_instruction=self.test_system_prompt, posthog_distinct_id="test-user"
+                model="gemini-2.0-flash", contents=contents, config=config, posthog_distinct_id="test-user"
             )
 
             self.assertEqual(len(self.client.capture.call_args_list), 1)